This invention relates generally to the treatment of photopolymeric resin containing mixtures obtained during photo-imaging processes, and more particularly to the treatment of spent mixtures produced in the removal of photoresist materials during the preparation of printed circuit boards. Still more particularly, this invention relates to treatment of such spent mixtures to reduce the gumminess and unctuousness of the mixture, thereby facilitating disposal of the spent mixtures.
Generally, preparation of a printed circuit board involves several steps. First the surface of a copper-clad substrate element of high dielectric strength is coated with a photopolymer film called photoresist. Next, a portion of the photoresist is exposed to electromagnetic radiation through a masking stencil. The radiation polymerizes the exposed areas of the photoresist, rendering the exposed areas less soluble. The photoresist coating of the copperclad substrate is then processed with a developing agent, conventionally a mild alkaline aqueous developer solution. While the developer solution is aqueous, it may contain a non-aqueous solvent, such as ethylene glycol ether, in addition to water. Upon contact of the coating by the developer solution, a reaction as illustrated in the following equation is understood to take place: EQU [R-COOH].sub.n +Na.sup.+ .fwdarw.[R-COONa].sub.n +H.sup.+
where R is a high molecular weight hydrophobic group.
The developer solution thereby removes the unexposed photoresist from the copper surface. By contrast, in the areas where exposure to light has polymerized or otherwise cured the film, changing the film's physical properties and lowering its solubility, the resist adheres to the copper surface and is not removed by the developer. The copper layer is then etched either with hydrogen peroxidesulfuric acid etchant or ammonical etchant to remove the copper in those areas unprotected by the mask of polymerized film remaining on the copper surface. After the polymerized photopolymer resin, or resist, has served its masking purpose, it is removed, or "stripped", by treatment with a stripping agent, normally a hot, alkaline aqueous or partially aqueous solution. Thus, a substrate with a partial coating of copper is formed. The copper remaining on the substrate forms a circuit pattern corresponding to the desired circuit configuration.
Another method for preparing printed circuit boards employs pasty, alkaline soluble photopolymers which, according to industry convention, are called "screening ink" or "liquid resist". In this method, a photoreactive emulsion is screened onto a copper coated substrate to form an image of the desired pattern. The pattern is then screened onto the substrate by forming the image on a taut screen made of stainless steel or synthetic fabric. The screen has a mesh size range from 200 to 230. The screen is aligned over the substrate and acts as a stencil. Ink is then applied by manually or mechanically pushing the ink across the screen with a squeegee. A thin layer (about 100 to about 300 microinches) of the resist film is spread thereby across the entire pattern. As a result, the image of the circuit pattern is transformed onto the copper coating on the substrate.
The screened image on the copper coating on the substrate is then cured by ultraviolet radiation or by baking. The curing process typically includes air-drying, pre-baking and post-baking in addition to the ultraviolet radiation or baking step. The pre-baking steps drive off some solvents, thereby avoiding run off of ink. The postbaking steps drive off residual solvents from the resist. The cured ink resists the chemical attack during the subsequent cleaning, plating and etching steps.
After the etching step, the ink is removed with an alkaline stripper solution. When the stripper solution is saturated with resist, the solution must be treated and fresh solution is added.
In either method of preparing circuit boards, when the developer solution and stripper solution have served their purpose of removing photopolymer resin, they are termed "spent." Typically in this process, the spent "solutions" have become saturated with resist, and often further contain undissolved resist (the photoresist and liquid resist are generically designated herein as "resist" or "resin"). Thus, commonly the spent "solutions" are not true solutions, but are actually heterogeneous mixtures comprising suspensions of particulate resins, often in colloidal form. The two types of spent mixtures, developer and stripper, differ from one another in that they contain photopolymers of different degrees of polymerization. Nevertheless, both types of spent mixtures require environmental treatment, typically by first separating undissolved photopolymer resins out of the spent mixture by filtration. The filtrate is then acidified to a pH of about 3 to 4 by addition of a mineral acid. Acidification precipitates formerly dissolved resins in the filtrate. The precipitated resins are then removed from the filtrate by a second filtration and the resulting substantially resin-free solution neutralized with alkali to an appropriate pH and treated as waste water. However, in the second filtration, the removed precipitated resins present serious problems since they are unctuous and gumlike, tenaciously adhering to any surface contacted. Thus, handling and disposal of the removed resins as well as cleaning of the equipment and other surfaces contacted by the resins is difficult.
An alternative treatment process may involve the addition of calcium chloride to the spent mixtures, as opposed to (not in combination with) mineral acid. The calcium chloride process utilizes what is called a "salt out" effect, in contrast to a chemical reaction. Such calcium chloride process also suffers drawbacks. For example, the removal of resist in such a process is not complete.